Logo: to the web site of Uppsala University

Materials with high electrical conductivity and large specific surface area are essential for biosensing, as they enable efficient charge transfer and enhanced performance. We present near-collector electroprinting, an electrostatic additive manufacturing technique for fabricating sub-millimeter biosensor structures with high surface area, improved conductivity, and feature sizes reduced by orders of magnitude. PEDOT:PSS was used as the base ink material, and Ti₃C₂ MXene was added to improve the electrical conductivity. The optimized ink achieved a conductivity of ∼117 S cm⁻¹. Using a nozzle with an inner diameter of 80 µm and optimized printing parameters enabled precise fabrication of well-defined multilayer structures. Separate lines were printed at 40 µm pitch, with top-layer fiber widths as small as 3 µm, demonstrating significant resolution enhancement relative to the nozzle size. The technique also enabled fabrication of solid structures at 30 µm pitch. Ink flow was evaluated over printing speeds of 0.5–4.5 mm s⁻¹ to identify instabilities and back-pressure effects. Functionality of the printed structures was assessed through protein attachment and release. Bovine serum albumin (BSA) release reached ∼198 ± 45 µg mL⁻¹, nearly a 107 ± 24% molar ratio relative to dextran sulfate. These results demonstrate strong potential for compact, high-resolution biosensing applications with efficient biomolecule interaction.

This thesis investigates the influence of increased layer thickness on the Electron Beam PowderBed Fusion processing of Ti-6Al-4V, with a focus on build productivity and the resulting material characteristics. Experimental builds were conducted on the Freemelt ONE system using several different layer thickness settings. For each build, the total process time was recorded and the resulting specimens were characterised in terms of surface quality, porosity, microstructure, and hardness.

The results show that increasing layer thickness directly reduces the number of layers required and therefore shortens overall build time. This demonstrates a clear productivity gain, although the extent of this improvement depends on the specific geometry and process conditions. All manufactured specimens successfully completed the build without defects that would require interruption or part abandonment.

Microstructural analysis revealed the expected α+β lamellar morphology characteristic of EB-PBF Ti-6Al-4V. Although grain sizes were not quantified, visual comparison indicates subtle differences between the layer-thickness conditions, particularly in lamella coarseness. Porosity measurements showed low overall pore content. Hardness testing produced values within the typical range for EB-PBF Ti-6Al-4V, with minor differences observed between samples.

Overall, the study confirms that increased layer thickness can be used to enhance process productivity while maintaining material quality within acceptable limits. The findings provide practical insight into the balance between manufacturing efficiency and material performance and highlight the relevance of parameter optimisation for EB-PBF processing of Ti-6Al-4V.

INTRODUCTION: Developing biodegradable implants is of growing interest to help fight antimicrobial resistance. Magnesium alloys are candidates for orthopaedic applications, but there is a need for improving their corrosion resistance[1]. Alloying reduces corrosion rates, but further improvement could be achieved by microstructural tuning using additive manufacturing (AM). However, process parameter optimization is time and resource consuming, motivating the use of numerical modelling to accelerate alloy development. The aim of this study was to predict the microstructure-temperature relationship for a MgCaZn alloy using the CALPHAD method as input for a classical nucleation and growth theory (CNGT) simulation. To achieve this, raw powder feedstock was characterized, along with a minimal number of printed samples.

METHODS: The powder composition was identified as Mg63.2Zn32Ca4.8 by Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES) and compared to the results of ToF-ERDA (Time-of-Flight Elastic Recoil Detection Analysis). The composition of printed samples was also measured to estimate evaporation rates during printing.

X-ray diffraction (XRD) was used for phase identification. Differential Scanning Calorimetry (DSC) was used to characterize phase transitions and generate Time-Temperature-Transformation (TTT) and Continuous-Heating-Transformation (CHT) diagrams.

RESULTS: Three cold crystallizations were distinguished (see Figure 1) and analysed by XRD. Kissinger analysis[2] was done on four heating rates and the results are shown in Table 1 together with the transformation onset temperatures. Metastable phases were formed at the first (Tp1, phase Mg51Zn20) and third crystallisation (Tp3, IM4). Thermodynamic data and unit cell parameters from experimental results and literature[3] were used in a Python code to simulate CHT and TTT diagrams for comparison with the experimental results. The experimental data was used to validate the numerical model.

DISCUSSION & CONCLUSIONS: The overall transitions and phases agreed with previous literature, with only significant differences in the obtained energies of activation. The first simulated TTT diagram to our knowledge has been generated for this composition and phases. Synchrotron studies and more compositions should be tested for proper validation of the methodology.

REFERENCES:

[1] Radha, R., & Sreekanth, D. (2017), J. Magnes. Alloy, 5(3), 286–312.

[2] Kissinger, H. E. (1957) Anal. Chem., 29(11), 1702–1706.

[3] Mezbahul-Islam, M. et al. (2014) Calphad, 46, 134–147.

ACKNOWLEDGEMENTS: This work was supported by the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by the Knut and Alice Wallenberg Foundation and the Swedish Research Council (grant numbers 2019-00191 and 2023-00155) supporting the Ion Technology Center.

Introduction

The use of biodegradable alloys in orthopaedic implants can reduce secondary surgical interventions, including those resulting from infections caused by bacteria attachment to inert implants. Mg alloys are promising materials, but a main challenge lies in improving their corrosion resistance to ensure their safe and effective use as large implants [1]. A widely explored system is the Mg-Zn-Ca ternary alloy. To achieve a further improvement in corrosion resistance of additively manufactured (AM) Mg-Zn-Ca alloys, a better understanding of the thermodynamic and kinetic phase transformations is needed. Hence, the aim of this study was to model the crystal nucleation and growth in Mg-Zn-Ca powders used for powder bed fusion with laser beam (PBF-LB), thereby providing critical insight towards designing more corrosion-resistant microstructures, while reducing the trial-and-error process that usually accompanies process parameter development.

Materials and Methods

Classical nucleation and growth theory (CNGT) was applied through an in-house Python code. CALculation of PHAse Diagrams-based (CALPHAD) thermodynamic simulations of the Mg-Zn-Ca system were used to predict steady state nucleation rates. X-ray diffraction (XRD) and Rietveld refinement was used to calculate cell parameters to use in the model. To validate our simulations of time-temperature-transformation (TTT) diagrams with experimental data, an extensive differential scanning calorimetry (DSC) study was realized, studying isothermal and variable heating rate thermal histories for different sized powders. The powder groups were three; denominated fine, medium and coarse, where the medium powder group had a normal distribution between 20 to 63 μm in diameter, which is typically used for printing. The fine and coarse contained particles out-of-range, namely those below 20 μm (fine) and those above 63 μm in size (coarse), typically considered waste.

Results

Constant heating rate DSC results revealed a difference in phase formation between powders with different size distributions, as shown in Fig. 1. Three common partial crystallizations are observed at around 150, 192 and 245°C with an additional peak at 290°C in the coarse powder, as well as an incongruent melting from 335°C. The additional peak in the coarse powder is also present in cast material in the literature [2]. The kinetics of these transformations were studied using the Kissinger method and were found to be in accordance with literature [3]. The experimental enthalpies of fusion were between 6.5 and 7 KJ/mol for all powders, which were substantially higher than expected from theoretical predictions. Results from the Rietveld refinement of the XRD patterns (Fig. 2) at different temperatures were comparable to previous studies on cast alloys [4]. Experimental points from isothermal DSC measurements overlapped with the simulated TTT diagram.

Conclusions

Experimental values from DSC and XRD allowed for the identification of the stable phases and their crystallization. As compared to cast material, there was one less transformation detected by DSC in fine and medium powders. The CNGT simulations gave good initial indicative results, but experimental validation was necessary for calibration of the model and to improve the accuracy of the predictions.

Aknowledgements

This work was partially supported by the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by the Knut and Alice Wallenberg Foundation.